Hundreds of Massachusetts residents may have been inadvertently exposed to measles, a highly contagious disease, while visiting two hospitals in Greater Boston last month, state health officials said Tuesday evening.

The alert covers anyone who may be showing early symptoms of the disease, which resembles a cold at first, and visited Massachusetts General Hospital in Boston between Aug. 17 -23 or MetroWest Medical Center in Framingham on Aug. 23 and 24.

Two people infected with measles, who apparently had no connection to each other, went to those hospitals for medical care, said Dr. Alfred DeMaria, an infectious disease specialist at the Massachusetts Department of Public Health. One was a child who had been too young to be vaccinated; the other was an older person who may not have been vaccinated.

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Officials said Mass. General patients were potentially exposed in the medical walk-in clinic Aug. 17 and 19, the emergency department on Aug. 20, and the inpatient areas between Aug. 20 and 23.

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Exposures may have occurred at MetroWest’s emergency department and inpatient areas on Aug 23 and 24.

The vast majority of people in Massachusetts are immune to measles because they have been vaccinated, or already had the disease, DeMaria said.

But the concern is for people who are not immune and may be at risk for serious complications, such as young children, pregnant women, and those with compromised immune systems, such as cancer patients.

“We don’t know who’s who, and there are potentially hundreds of people who may have been exposed at these places,” DeMaria said.

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Measles is spread by coughing, and particles of the virus can float in the air for up to two hours, DeMaria said.

At Mass. General, officials identified roughly 300 people who may have been exposed, said Dr. David Hooper, the hospital’s chief of infection control.

The hospital has been contacting those people and offering them a blood test that can quickly determine whether the person is immune. Hooper said those not immune were offered a vaccine.

The Health Department is advising anyone who may have been exposed and begins to develop symptoms — fever, cough, runny nose, red eyes — to call their health care provider before visiting an office, clinic, or emergency department because that visit may put others at risk of exposure.